In diverting and rebuilding 2.1 km of minor road for UK COAL’s new Steadsburn surface mine, Tensar TriAx™ geogrid and Tensar Geocell foundation mattress technology is aiding support services and construction company Carillion PLC to achieve a stabilised foundation over old backfill with savings in cost and time, as well as low ecological impact.
”Tensar’s geogrid solutions offered the optimum combination of performance, cost effectiveness and excellent sustainability in a very complex situation,” says David Lindsay of consulting engineers Scott Doherty. “The main difficulty was the potential for differential settlement between the old surface mine, which had been back filled with a variety of uncompacted materials, and the virgin land. There was also the added possibility of groundwater being rebound even after it had been pumped off.
“Concrete slab and piled solutions would have been very expensive in this project and taken extra time and materials. I have worked with Tensar on previous occasions, and have found their extensive technical support and professional attitude very helpful.”
The Steadsburn Surface Coal Site at Widdrington, Northumberland is being worked by UK COAL for coal and fireclay. The operation involves diverting a short section of the C115 road and the removal of a former processing depot to gain access to the remaining shallow coal on site.
The ground traversed by much of the diverted road comprises uncompacted backfill over the previous surface workings which are up to 85m depth. There is a significant risk of reactivating differential settlement between the virgin unworked ground and the backfill. Fault lines, shafts from underground pits and ground water and water table variations exert further stress on the fill material.
After considering conventional techniques, including a 300mm thick continuous cast reinforced concrete slab for the road foundation, as well as a pile supported concrete slab, Scott Doherty worked with Tensar to design a solution based on geogrid technology which offered considerable cost, time and sustainability benefits.
The solution adopted at Steadsburn uses Geocells foundation mattresses constructed as a continuous cell structure from geogrid and containing granular material to create a stiff platform for overlying construction at the edges of the backfilled void. These are teamed with TriAx™ geogrid reinforcement in the roadway sub-base to control differential settlement, and prevent the sharp surface profiles, typical of settlement.
“Two Geocell mattresses are being constructed, one 100m by 12m by 1m deep, the other 80m long, where the road runs over the boundaries between virgin ground and the poor and variable backfill,” explains Martin Forster of groundwork contractors Owen Pugh Contracts Ltd. “The Geocells are being filled by site won granular spoil, to the DoT 6F3 capping material standard, sourced from the material planed from the existing road.
“TriAx™ geogrid is being used in a basal and an upper layer in the capping over a 1.6 km length of road, with compacted site won 6F2 material. It is installed within the embankments supporting the road, over the Geocell matresses and, also, as part of the main road construction over the general backfill.”
Overseeing Project Engineer for Northumberland CC, John Thompson, adopted the use of Tensar geogrid solutions at the contract design stage. He says: “By allowing the use of locally planed aggregate and concrete rubble from the Widdrington Disposal Point, the Geocell mattress and the TriAx™ provides a stable road foundation which allows us to reduce the capping layer, and provides a highly sustainable solution by recycling spoil and reducing transport movements.”
Colin Thompson, Key Accounts Manager for Tensar International concludes: “Tensar have worked closely with the consulting engineer and the clients, to demonstrate the value of their solutions in performance, cost savings and reducing the ecological impact of this major construction project”
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